Process for purifying phosphoric acid



Search Room Cross Reference CHEMISTRY Filed Aug. 28, 1936 2 Sheets-Sheet l 01W 0 Y R m m A $3; a H ww M w m MUN m W fi m a W 3030 QR m w 3% S 0 0 w 1 m. m 2 mm M y M -Q-U NI. 0 W L 88 Wm W W w fl .mJuOoWQ H 9m W NM. w WNM w 1 as W We 5 e i mN MN mw MN N u NN NN NN 8k ow mowfiw flwlwu m. Q l e 8.8 0 Q UQQ HWQS uh P w W Q g m W@ QQLS B em he NEH bearcn noom .CHEMLSTRY, Cross Reference July 4, 1939. A. B. HETTRICK PROCESS FOR PURIFYING PHOSPHORIC ACID Fi led Aug 28, 19256 2 Sheets-Sheet 2 1 1 lrleeeem #m a //M lll ll 201.9 18

Patented July 4, 1939 UNITED STATES PATENT OFFICE PROCESS FOR PUgaFnglNG PHOSPHORIC Ames B. Hettrick, Piney River, Va., assignor, by

mesne assignments, to Virginia Chemical Corporation, a corporation of Delaware Application August 28', 1936, Serial No. 98,310

3 Claims. (Cl. 223-165) This invention relates to process and appameans of non-metallic difiusion units while held ratus for purifying phosphoric acid, and more in an acid-proof brick-lined tank.

particularly to the removal of fluorine compounds The process of fluorine removal is most easily from phosphoric acid by difiusing a heated gas carried out by maintaining the acid at about 280 through the acid. to 290 F. In the preferred process, saturated 5 In the accompanying drawings which illussteam at suflicient, pressure to bubble it through trate preferred embodiments of the invention, the tank of acid is used and this is passed through Figure 1 is a diagrammatic illustration of the an individually fired superheater before it enters Whole apparatus; the tank. Sufiicient heat is added to raise the Figure 2 is a horizontal section through the temperature of the steam from saturated temfluorine removal tank taken on the line II-II perature at 5 to 8 pounds per square inch gauge of Figure 3, showing the diffusion units; to about 700 F. Bubbling this superheated vapor Figure 3 is a vertical section through the fluothrough the concentrated acid serves two purrine removal tank taken on the line III-III poses:

of Figure 2; (1) To transmit the heat required to main- Figure 4 is a detail vertical section taken on tain the required temperature from the heat the line IVIV of Figure 2, illustrating a difsource to the-batch of acid. fusion unit and a portion of the surrounding lin- (2) To serve as a gaseous medium for volatilizing of the tank; and ing and removing the fluorine compounds.

Figure 5 is a detail vertical section of a modi- For efiicient operation it is necessary that the 20 fled form of difiusion unit, the view also showing steam be diffused through the acid as thorthe associated parts. oughly as possible, preferably as minute bubbles.

Concentrated phosphoric acid as made by any This may be accomplished by using difiusion of the well known processes such as the wet suldisks of carbon or silica. These materials are phuric acid, electro-thermal, or the blast furused in preference to metallic diffusion devices 25 nace methods contains a considerable amount of because the temperature prevailing, together fluorine in one or more of its various comwith the presence of fluorine and phosphoric acid, pounds. These concentrated acids contain fluohave proven to be very corrosive towards all rine in amounts varying from 800 to 3000 parts metals, even those ordinarily considered to be per million. highly corrosion-resistant. 3

This fluorine content is too high to permit use The unique advantage in the use of superof this concentrated phosphoric acid in making heated steam is that most of the steam is used food grade mono-calcium phosphate without iuras a gaseous medium and only a small portion ther purification. Boiling this concentrated phosis condensed. This condensation takes place durphoric acid at elevated temperatures for long peing the period that the acid is being heated up 35 riods in order to eliminate fluorine compounds to its boiling point. The latent heat liberated therefrom has been found to be unsatisfactory by this condensing steam assists in raising and when it is attempted on a commercial scale.- maintaining the temperature and the condensed This difficulty in using this procedure in plant water prevents further concentration of the practice has been in obtaining materials of constrong acid. Thus there is a continuous cycle of 40 struction which would permit boiling of acid on evaporation and condensation. The heat apa large scale. It is not practical to use metallic plied to the saturated steam during the supersteam coils or lining for the tank in which the heating process is transmitted to the acid in the boiling is carried out as the strong phosphoric tank and serves to raise and maintain the temacid, plus the fluorine compounds being driven perature at the desired point, but at the same 45 off during the boiling process, are exceedingly time the condensation in the tank during the corrosive. For example, a 3000-gallon batch of heating up period is sufi'icient to maintain the acid was boiled for 72 hours at 260 F. in a leadconcentration preferably at about 70 to 72% lined tank with a lead steam coil and approxiduring the entire boiling cycle of 48 to 72 hours.

50 mately 50 pounds of lead was dissolved and taken This is highly desirable as there is no possibility 50 into solution. of increasing the concentration and consequently I have invented a process whereby the use of little possibility of increasing the temperature any metal in contact with the acid is eliminated to the point where there is danger of converting by blowing finely diffused superheated steam or the ortho-phosphoric acid to the pyro-phosphoric air or oth r suit ble g thro gh the acid by acid form. Air may be used as the heat transfer 55 medium, but superheated steam is preferred because of, its higher specific heat.

The chief virtue of this process is that it can be usedtoremove fluorine without contaminating the acid with any metallic impurities which would have to be removed subsequently. For example, the acid before? treatment for' removal of fluorine contains approximately 1 part per million of lead, and is in all respects suitable for use in manufacturing food products Without further process- 2 ing for the removal of lead or other metallic impurities. However, if lead coils are used for boiling the acidfor fluorine removal, the lead content of the acid will increase to several thousand parts per million. This is true, to a more or less degree, with all the common available mate'- rials of construction including the alcidresisti'ng alloys. With my process, which is carried out in an acid-proof brick-lined tank with the super heated steam or air injected through carbon or silica tubes and porous diffusion disks, I am able to produce a concentrated acid of low fluorine content without increasing the' metallic contamination and no further processing is required for removal of metallic contamination.

This process may be carried out either with or without having silicon compounds present. In this process silicon compounds maybe present in the acid fed to the process and additional silicon compounds are dissolved in the acid through the slight reaction of the hot acid with the acid-proof brick lining.-

My process has been used for purifying several thousand gallons of acid and the fluorine content of the acid has been reduced in all cases from 1000 to 1200 parts per million to below 50 parts per million in periods of 48 to 72 hours.

Referring more particularly to the accompanying' drawings, and for the present to Figure l, saturated steam from any suitable source is conducted through a pipe 2 provided with a reducing pressure valve 3, gauges 4 and a safety valve 5 to a superheater 6. In the superheater, the steam passes through a coil 7 and is heated by a burner 1a to a temperature of about 400 to 700 F., preferably about 700 F. The superheated steam is conducted through a pipe 8 provided with a valve 9 to a header! which distributes the superheated steam through pipes I provided with valves 2 to branch headers l3 which are connected to tubes |4' connected to diffusion units I5 located in the fluorine removal tank |6.

Referring now to the other figures, the side of the fluorine removal tank is made of wood staves I! and the bottom |8 is also of wood. Inside of this is placed a' lead lining l9. Inside of the lead lining is placed a lining 20 of acid-proof brick which covers the side and bottom of the tank. An inner lining 2| of acid-proof brick is placed in the bottom of the tank on the lining 20. The lining 2| of acid-proof brick is provided with openings 22, in the embodiment shown, seven such openings being provided, each for the reception of a filtering disk or unit 23. The bricks forming that portion of the lining 2| adjacent the openings 22 are recessed as indicated by the reference numeral 24 so as to receive the diffusion units. The diffusion units are cemented to the lining 2| with acid-proof cement 25. Each diffusion unit has an opening 26 extending through it, into which extends a tube 27 which is cemented to the diffusion unit by acid-proof cement indicated by the reference numeral 28. 29 provided between the diffusion unit 23 and the lining 20, so that superheated steam .or air conducted to the chamber through the tube 21 will have an opportunity to diffuse through the diffusion disk 23 into the acid. The diffusion unit 23 and tube 2! are made of a non-metallic acid-- proof material such as silica or carbon. The diffusion disks are porous, but the tubes 21 are impervious. The tubes are further protected from attack by the acid. by acid-proof bricks or beads- 30. The tubes extend upwardly through the tank which is provided with a lead cover 3|, the upper ends of the tubes being connected to the superheated steam or air supply headers. Asbestos packing 32 is placed between the tubes 21 and the holes in the cover through which the tubes extend in order to prevent possible breakage of the tubes and to seal the openings. Such packing is particularly desirable if instead of making the tubes of silica or carbon, they are made of heat resisting glass. The fumes driven off from the acid are carried away through a stack 33.

In Figure 5, a different form of porous diffusion unit is shown. Instead of being in the form of a flat disk or plate, such as shown in the other figures, it is of hemispherical shape. It comprises a flat base 35 and a hemispherical top 36. The base is cemented to the bottom lining 31 of the tank by a'cidproof cement 38. superheated steam or air is supplied to the difi'usion unit through an impervious carbon pipe 39 also cemented to the unit as indicated by the reference numeral 40. In this embodiment, the diffusion unit itself forms a chamber 4| for the superheated steam or air which is diffused through the acid in the removal of fluorine and its compounds.

The term fluorine as used in the specification and claims, is intended to cover not only fluorine per se, but also its compounds.

I have illustrated and described two embodiments of the apparatus and the preferred procedure in carrying out the process. It is to be understood, however, that the invention may be otherwise embodied or practiced within the scope of the following claims.

I claim:

1. A process of reducing the fluorine content of concentrated phosphoric acid which comprises blowing superheated steam up through the boiling acid, kept at a temperature of approximately 280 F. for a period of time suflicient to reduce the fluorine content to less than 50 parts per million, while preventing the evaporation of water beyond the original concentration of the acid.

2. A process of reducing the fluorine content of concentrated phosphoric acid, which comprises diffusing superheated steam through the boiling acid kept at a temperature above the boiling point of the acid for 48 to 72 hours while preventing evaporation of water substantially beyond the original concentration of the acid.

3. A process of reducing the fluorine content of concentrated phosphoric acid, which comprises diffusing superheated steam through the boiling acid kept at a temperature above the boiling point of the acid for a period of time suflicient to reduce the fluorine content to less than 50 parts per million while preventing evaporation of water substantially beyond the original concentration of the acid.

AMES B. HE'I'IRICK.

There is a space 

